: The recent discovery of HIV coreceptors has provided a new and dynamic vision of HIV envelope protein function. The PI and his colleagues have expanded on this concept to propose that the transient envelope protein interactions and conformational changes that arise during CD4- and coreceptor-mediated binding and fusion may expose critical targets for neutralizing antibodies, especially antibodies capable of neutralizing primary isolates (PI) of HIV. In this regard, they have developed novel "fusion competent" vaccines comprising cocultures of cells expressing HIV envelope protein and cells expressing CD4 and CCR5 coreceptor, captured during binding and fusion. The PI and his colleagues have tested these complex vaccines for immunogenicity in a unique transgenic mouse vaccination model. In marked contrast to results using current HIV vaccine immunogens that present only static envelope protein, they have observed potent neutralization of Primary Isolate virus using sera from mice presented with our functioning envelope protein. To refine these immunologic observations, and to study the structure of the HIV envelope during binding and fusion, they developed a high affinity S-peptide "tag" to facilitate the isolation of CD4-associated complexes. S-protein affinity chromatography will be used to isolate chemically crosslinked complexes, including CD4-coreceptor and CD4-envelope coreceptor. Components will be identified using specific monoclonal and polyclonal antibodies. Affinity-purified complexes will be tested in the transgenic mouse vaccination model to identify those structures critical for engendering Primary Isolate virus neutralizing antibodies. Monoclonal antibodies will be derived from these studies to further dissect fusion-dependent epitopes and vaccine targets. This project may define the structural components in HIV binding and entry, and may suggest developmental approaches to an effective and practical HIV vaccine, and addresses the targeted area of "structure and immunogenicity of HIV envelope proteins."